1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the device, particularly to a semiconductor storage device using a ferroelectric film, and a method for manufacturing the device.
2. Description of the Related Art
In recent years, a ferroelectric storage device (FeRAM: ferroelectric random access memory) using a ferroelectric capacitor has been noted as one of nonvolatile semiconductor storage devices.
In semiconductor devices such as a ferroelectric storage device in which a ferroelectric film is used in a capacitor, if hydrogen penetrates the capacitor in a manufacturing process after forming the ferroelectric capacitor, characteristic of the capacitor, especially polarization characteristic of the ferroelectric film is deteriorated. To prevent the deterioration, the capacitor is covered with a hydrogen barrier film, for example, an aluminum oxide film (Al2O3 film), which protects the penetration of hydrogen.
One example of a ferroelectric storage device by a conventional technique is disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 2001-237393. The semiconductor device in the example includes: a MOSFET (metal oxide semiconductor field effect transistor) 2 formed on a semiconductor substrate 1; an insulator 4 formed on the MOSFET 2; a first hydrogen barrier film 5 disposed on the insulator 4; a ferroelectric capacitor constituted of a lower electrode 7, a ferroelectric film 8 and an upper electrode 9 disposed on the first hydrogen barrier film 5; and a second hydrogen barrier film 10 covering the ferroelectric capacitor. The second hydrogen barrier film 10 is brought into contact with the first hydrogen barrier film around the ferroelectric capacitor. Thus, when each ferroelectric capacitor is covered with the first and second hydrogen barrier films 5, 10, penetration of hydrogen into the ferroelectric capacitor is prevented.
Moreover, a structure in which a whole ferroelectric capacitor cell array constituted of a plurality of ferroelectric capacitors is covered with a hydrogen barrier film has been reported in “0.18 um SBT-based Embedded FeRAM Operating at a Low Voltage of 1.1V”, by Y. Nagano et. al., 2003 Symposium on VLSI Technology Digest of Technical Paper. A semiconductor device in the article includes: a plurality of ferroelectric capacitors formed on a first hydrogen barrier film; an interlevel insulator which is formed to cover the plurality of ferroelectric capacitors and which is divided for each ferroelectric capacitor cell array; and a second hydrogen barrier film which covers the ferroelectric capacitor cell array including the interlevel insulator. The second hydrogen barrier film is brought into contact with the first hydrogen barrier film around the ferroelectric capacitor cell array.
In any of the above-described cases, the second hydrogen barrier film is simultaneously formed on upper and side surfaces of the ferroelectric capacitor or the ferroelectric capacitor cell array. The hydrogen barrier film formed on the side surface is generally inferior in film quality and step coverage as compared with the hydrogen barrier film formed on the upper surface which is a horizontal face. For example, even when the hydrogen barrier film is formed by atomic layer deposition (ALD) that is said to be a method achieving better step coverage, a film thickness on the side surface is about 70% of that on the upper surface. Therefore, it is hard to make a barrier capability against the penetration of hydrogen from the side surface to be equal to that from the upper surface.
Therefore, there has been a need for a semiconductor storage device comprising a ferroelectric capacitor superior in barrier capability against penetration of hydrogen from all directions including a transverse direction, and a method for manufacturing the device.